This invention relates to high-speed data transmission over a twisted pair local access connection. It is specifically concerned with matching of power spectra density of a high rate data channel with a transfer function of a twisted pair.
High-speed data transmission over a twisted pair (i.e., subscribers local access line) such as HDSL, as it is presently carried out, uses transmission procedures full of complexity. Such complexities include multicarrier modulation, multitone transmission and orthogonal frequency division. These processes are all susceptible to external interference and to crosstalk.
Transmission of signal s(t) in a Twisted pair is subject to various types of interference including near end crosstalk (NEXT), which affects high speed data transmission. Multiple channels may be transmitted over a single twisted pair, but may interfere with one another. Twisted pair channels must be substantially orthogonal to one another in order to limit interference between channels.
A functional diagram of these interferences is shown in the FIG. 1A, where an input lead 101 represents application of the data signal s(t) to the twisted pair. The twisted pair may be characterized by a transfer function that is related to the absolute square of the functional value Hc(f), which represents an attenuation characteristic at the twisted pair, and is proportional to {square root over (f)}. In addition, there is significant interference from NEXT, represented by Ha(f). Far end crosstalk, FEXT, is relatively very low compared to NEXT and is not included in the model.
The input and the NEXT are applied to mixer 107 that represents the interaction of the signals. The output on lead 109 represents the amalgam of the data signal plus the interference signals.
The effect of this interference of the twisted wire attenuation of signals is shown by the graph of FIG. 1B where coordinate axis 100 of the log of signal frequency is plotted against a coordinate axis 102 representing attenuation in dB.
Curve 111 represents the attenuation of the data signal as function of frequency. Curve 113 represents the increase of NEXT as frequency increases. It is apparent that as the data signal attenuates with increasing frequency, and the interference signal increases with the increasing frequency. NEXT is a dominant portion of this interference. Other forms of interference include narrow band radio interference. All these interferences contribute to the frequency limits of the twisted pair. In a typical instance 95% of twisted wire capacity is below 10 kHZ and 60% of capacity is below 40 kHZ.
A plurality of data signals are separated into parallel bit streams, with each parallel stream having a bandwidth characteristic such that the combined cumulative effect of all the signals with individual bandwidths produce spectral characteristics of the data signals that can accommodate and emulate the spectral high speed data transmission characteristic of a twisted pair.
In a particular embodiment parallel data streams resulting from a serial to parallel conversion of an input data stream and multiplexing into a number of parallel symbol streams, including data transmitted at different data rates are each individually spread to a different bandwidth so that the combined effect of the selected bandwidths has a spectral distribution resembling the spectral transmission characteristic of a twisted pair.
A proposed embodiment based on Orthogonal Code Division Multiplexing (OCDM) performs two essential steps: (1) performing a serial to parallel conversion of a serial input into a plurality of orthogonal serial streams each having different chip rates, and (2) loading of each of the paralleled streams by adaptive modulation. These two steps are combined with spreading code generation techniques to achieve high-speed data transfer over twisted pair.
Each individual paralleled stream is spread to a different bandwidth, with the narrowest bandwidth having the highest modulation loading and the broadest bandwidth having the lowest modulation loading. The selected modulation technique in one embodiment is pulse amplitude modulation with no carrier. Adaptive loading of the signal streams (loading refers to a matrix dimensionality of a modulating signal) is selected and applied to distribute the power spectra so that it matches the twisted pair transfer function. Each stream may represent different services.
Fundamentally the concepts of the OCDM scheme is a serial-to-parallel conversion (i.e., multi-codes) and overspreading (i.e., spectral matching). An input data stream is serial-to-parallel converted to parallel branches. Each branch is transmitted with spreading orthogonal codes applied. The number of parallel branches N is equal to the spreading gain of these orthogonal codes. Spectral matching to the channel is preceded by an overspreading of the chips of the spreading orthogonal code. Overspreading is recursive with a (+,xe2x88x92) pattern. Different powers are assigned to different levels. Different levels of overspreading are orthogonal to each other. Serial-to-parallel codes may be reused in spreading and overspreading in different channels of overspreading according to a spectral matching desired.
An advantage of the coding scheme is that it is effective in rejecting radio interference even with unshielded twisted pair.